A fuel grade alcohol production process, e.g, for the production of ethanol, typically includes fermentation of a mixture of water and milled grain to yield alcohol, distillation of the fermented mixture to recover alcohol as a top product and distillery bottom by-products, which includes grain solids and thin stillage of dissolved solids in water. The distillary by-products are typically concentrated by evaporation of water therefrom, to yield Distiller's Dried Grains with Solubles (DDGS), a valuable feed for livestock.
To make fuel grade alcohol production more economical, it is desirable to reduce the external energy and water required to operate the various steps in the alcohol production process. This can be achieved, for example by integrating the waste heat of one unit operation as a heat source for use in another unit operation of the process, process to process heat exchange, and recycling waste water streams back into the process. For example, U.S. Pat. No. 7,297,236 to Vander Griend describes an ethanol production process in which the steam generated from concentrating the thin stillage can provide heat for operation of the distillation of the fermented mixture.
In the ethanol production process described in U.S. Pat. No. 7,297,236, a series arrangement of four first effect evaporators and four second effect evaporators concentrate the thin stillage, and the second effect steam from the second effect evaporator is used to operate the distillation portion of the process. An overview of an exemplary conventional dry grind ethanol plant 100 incorporating four first effect evaporators and four second effect evaporators, such as that described in U.S. Pat. No. 7,297,236, will be described with reference to FIG. 1. As shown in FIG. 1, ethanol plant 100 includes a fermentation portion 110 where hot water 104 and milled grain 102 (e.g., corn) are mixed to form a mash, cooked, and fermented by yeast in a fermentor to yield a fermented feed 106. Fermented feed may be sent to a degasser (not shown) to remove any non-condensable gases and then separated in a beer column 120 into an overhead ethanol-rich vapor 108 (e.g., 120 proof) and beer bottoms 116. The non-condensable gases from the degasser may be further processed to recover any ethanol as a condensate (not shown) that can be fed back to a beer column 120, and the gas sent to a scrubber (not shown). The scrubber water (not shown) may be recycled to the fermentation portion 110 of the process.
The ethanol-rich vapor 108 from the beer column 120 enters a rectifier column 130 where ethanol vapor having a higher concentration of ethanol (e.g., 190 proof) is generated as an overhead vapor 132. Steam 129 from heating the milled grains and water in fermentation portion 110 may also be fed to a stripping portion of the rectifier column to assist in stripping ethanol in the liquid at the column bottom. The 190 proof ethanol vapor 132 is condensed and dehydrated with heaters in a molecular sieve at portion 140 to yield a high grade ethanol vapor product 112 (e.g., 199.5 proof). Ethanol vapor product 112 may then be cooled and condensed by a cooler/condenser 145 to yield a liquid ethanol product 112″. The molecular sieve may be regenerated by removing the absorbed water, which can include some ethanol. The removed water may be cooled/condensed by a regenerate cooler/condenser (not shown) and returned to the rectifier 130 via regenerate stream 114. If non-condensible gases are present in the rectifier column, these gases may be recovered and also sent to the scrubber (not shown).
Beer bottoms 116 from the beer column 120 containing mostly water, dissolved materials and unfermented solids from the milled grain, may be sent to a centrifuge 160 and separated into a mostly solid component known as distiller's grains 172 and a mostly liquid component known as thin stillage 118. A portion 118′ of the thin stillage may be reintroduced into the fermented mixture at fermentation portion 110, and the remainder sent to an evaporation portion 150 of the plant. In the evaporation portion 150, water is evaporated from thin stillage 118 to produce a syrup 158. The evaporation portion includes four first effect evaporators 151, 152, 153 and 154 connected in series (via respective lines 157) and four second effect evaporators 161, 162, 163 and 164 connected in series (via respective lines 157). The first three evaporators 151, 152 and 153 of the first effect are operated using plant steam 190 as a heat source for evaporating water from the thin stillage, and the fourth first effect evaporator 154 is operated using ethanol vapor 112′ taken from the ethanol vapor product stream 112. The first effect evaporators incrementally evaporate water from the thin stillage to produce mid stillage 156. Mid stillage 156 is sent to the first second effect evaporator 161, and then in series to the subsequent second effect evaporators 162, 163 and 164 that incrementally evaporate water from the mid stillage to produce syrup 158. Syrup 158 can be added to the distiller's grains 172 in a mixer 170 to produce a mixed feed 174 that is dried in a distiller's grain dryer 180 to yield DDGS 182.
The second effect evaporators are operated using first effect steam 192 generated in the first effect evaporators. Second effect steam 194 generated in the second effect evaporators is delivered to provide heat for operation of the beer column 120. Steam condensate from the evaporators is discharged through a condensate line (not shown) and may be heated and recycled to the fermentation portion 110 of the process. U.S. Pat. No. 7,297,236 describes providing valves on the various lines leading to the evaporators so that any one of the four first effect evaporators 151, 152, 153 and 154, and any one of the four second effect evaporators 161, 162, 163 and 164 can be taken off-line and by-passed for maintenance.
In recovery processes for other alcohols, the use of second effect steam as described in U.S. Pat. No. 7,297,236 may not be an efficient integration of waste heat. In addition, the production of other alcohols may not yield a superheated alcohol vapor product that can be integrated as a heat source for the evaporators, as in ethanol plant 200 and the ethanol process described in U.S. Pat. No. 7,297,236. For example, butanol is an alcohol with a variety of applications, such as use as a fuel additive, as a blend component to diesel fuel, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Butanol is favored as a fuel or fuel additive as it has a higher energy density than ethanol and yields only CO2 and little or no SOX or NOX when burned in the standard internal combustion engine. Additionally, butanol is less corrosive than ethanol, the most preferred fuel additive to date. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means. As the projected demand for butanol increases, interest in producing butanol from renewable resources such as corn, sugar cane, or cellulosic feeds by fermentation is expanding.
Butanol production can be less energy efficient than ethanol production for a given milled grain load. The fermented mixture in a butanol production process typically has a lower concentration of butanol because of butanol's toxicity to the butanol-producing microorganisms in the fermentor. In a fermentative process to produce butanol, in situ product removal advantageously reduces butanol inhibition of the microorganism and improves fermentation rates by controlling butanol concentrations in the fermentation mixture. Technologies for in situ product removal include stripping, adsorption, pervaporation, membrane solvent extraction, and liquid-liquid extraction. In liquid-liquid extraction, an extractant is contacted with the fermentation mixture to partition the butanol between the fermentation broth and the extractant phase. The butanol and the extractant are recovered by a separation process, for example by distillation. In the recovery process, the butanol can also be separated from any water, non-condensable gas, and/or fermentation by-products which may have been removed from the fermentation broth through use of the extractant. Thus, butanol production may include unit operations, absent in ethanol production, for recovering butanol from the butanol-containing extractant phase. Moreover, in the production of butanol, a distillation portion of a butanol recovery process may not yield a hot butanol vapor product. Also, second effect steam may be more heat than is needed to operate the distillation columns for butanol recovery.
What are needed are systems and methods for recovering alcohol, and in particular butanol, that are energy efficient while still being flexible in operation. The present application satisfies these and other needs, and provides further related advantages, as will be made apparent by the description of the embodiments that follow.